The fundamental standard of time in the United States is provided by a cesium beam resonator whose accuracy is of the order of one part in 10.sup.13. By radio broadcasting signals related to the frequency of the cesium beam standard, the National Bureau of Standard makes the frequency and time standards available to organizations equipped to receive the signals. Atomic clock accuracy is needed for various systems such as telecommunications, electronic navigation systems, and scientific measurements. Such organizations utilize a secondary clock which can be synchronized with the national standard. Such secondary clocks are operated independently and are physically remote from the national standard. Synchronization is typically made daily by radio and occasionally by the use of portable cesium beam clocks which are transported to the secondary clock locations.
An instrument widely used as a secondary clock is the electronic counter-timer which uses a piezoelectric crystal oscillator. The crystal oscillator frequency is very stable since the crystal is kept in a temperature-controlled oven. A typical operating frequency is 10.sup.7 Hz while the drift in frequency may be of the order of 7 parts in 10.sup.10 per week. This gradual drift can, over a period of time, cause significant errors; thus, highly accurate measurements require periodic recalibration of the local oscillator against a suitable primary standard such as the National Bureau of Standards radio broadcast signal.
Generally, crystal ocsillators are subject to three types of frequency instabilities, a linear change in frequency with time known as aging drift, a non-linear change in frequency with ambient temperature known as thermal drift and a random frequency fluctuation including flicker noise. These instabilities will cause the secondary clock to lose synchronization with the master clock. Aging drift and ambient thermal drift are predictable and therefore removable, but flicker noise is unpredictable and therefore not removable. Accordingly, periodic synchronization with a primary standard cannot be avoided. There is a continuing interest in improving the stability of the freerunning operation of the resonator during the intervals between calibrations.